The government sues Standard & Poor's, saying it improperly gave high marks to mortgage securities that failed and sparked the financial crisis.

Homes on the market in the Antelope Valley in 2007 at the start of the subprime… (Brian Vander Brug, Los Angeles…)

The federal government is embarking on one of its most ambitious efforts to assign blame for the financial crisis, going after Wall Street's biggest credit rating firm for its role in pumping up the housing bubble.

The action marks the first federal crackdown against a major credit rater, and it signals an untested legal tack after limited success in holding the nation's banks accountable for the part they played in the crisis.

The government selected Los Angeles as the venue to file the lawsuit in part because it was one of the regions hardest hit when the bottom fell out of the housing market. Hundreds of thousands of California residents lost their homes to foreclosure, and others saw their wealth evaporate as properties plummeted in value.

"The DOJ is playing hardball and they're coming at the ratings agency in a very different direction with a potentially very powerful weapon to push S&P to the settlement table," said Jeffrey Manns, a law professor at George Washington University.

In addition to the Justice Department, several state attorneys general are investigating the ratings agency. States such as California and New York are expected to pursue their own investigations and legal action, people familiar with the matter said.

S&P has faced other lawsuits from investors and the states of Illinois and Connecticut.

California is expected to sue S&P under the state's False Claims Act, one person familiar with the matter said. The law makes it a crime to defraud the state, and damages of up to three times the amount of the claim can be awarded if the victim was an institutional investor, such as one of the state's pension funds.

The federal action does not involve any criminal allegations. Critics have complained that the government has yet to send any senior bankers or Wall Street executives to jail for potential illegal behavior that led to the crisis.

But civil actions typically require a much lower burden of proof.

Investors rely in part on rating agencies to decide what stocks, bonds or other securities to buy based on the agencies' recommendations about their safety. The three major raters – S&P, Moody's Investors Service and Fitch Ratings — have all been criticized for giving perfect AAA ratings to complex bonds in 2007 that later turned out to be nearly worthless.

It was not known why Standard & Poor's was singled out in the federal lawsuit.

The government and S&P have tangled before. The rating agency in August 2011 issued a historic downgrade of U.S. creditworthiness and threatened to lower it even further.

The two sides were reportedly in settlement talks that broke down during the past week. The ratings firm could face hundreds of millions of dollars in fines and new restrictions on its business model if found liable of civil violations.

S&P, which is a unit of publisher McGraw Hill, denounced the lawsuit in a detailed and strongly worded response. The company said the claims were unjustified, adding that it acted in "good faith" to warn the world about some of the securities that went belly up.

"A DOJ lawsuit would be entirely without factual or legal merit," the company said, adding that even the U.S. government "publicly stated that problems in the subprime market appeared to be contained."

The rating firm has steadfastly maintained that it was protected under the 1st Amendment to state an opinion about certain financial products. That argument may not hold up if federal or state investigators are able to prove that the ratings agency knowingly gave improper evaluations.

The lawsuit zeros in on a series of collateralized debt obligations that were created at the height of the housing boom in 2007, according to S&P. The value of these exotic mortgage securities was nearly wiped out when the subprime mortgages they were tied to imploded.

Lawrence J. White, an economics professor at New York University's business school, believes that the housing crisis could have been more contained if ratings agencies had been more careful.

"If they had been more conservative in their ratings, fewer bonds would have been sold, the interest rates would have been higher, fewer mortgages would have been granted," White said. "There would still have been a housing bubble, but it might not have been quite so severe."

When U.S. housing prices plummeted by 35% — sucking about $7 trillion from home values nationwide, according to White — the decline spilled over into the financial sector, which had gobbled up the toxic mortgage bonds as well as enormous amounts of debt.

The crisis wound up toppling Wall Street giants such as Bear Stearns and Lehman Brothers, sending markets into turmoil.